Abstract

An earthquake is a sudden rupture process in the Earth’s crust or mantle caused by tectonic stress. To understand the physics of earthquakes it is important to determine the state of stress before, during, and after an earthquake. There have been significant advances in seismology during
the past few decades, and some details on the state of stress near earthquake fault zones are becoming clearer. However, the state of stress is generally inferred indirectly from seismic waves which have propagated through
complex structures. The stress parameters thus determined depend on the specific seismological data, methods, and assumptions used in the analysis,and must be interpreted carefully.
This paper reviews recent seismological data pertinent to this subject,and presents simple mechanical models for shallow earthquakes. Scholz(1989), Brune (1991), Gibowicz (1986), and Udias (1991) recently this subject from a different perspective, and we will try to avoid duplication
with these papers as much as possible. Because of the limited space,available, this review is not intended to be an exhaustive summary of the literature, but reflects the author’s own view on the subject.

"Reprinted, with permission, from the Annual Review of Earth and Planetary Sciences, Volume 22 copyright 1994 by Annual Reviews, www.annualreviews.org"
I benefited from discussions with Tom Heaton, Don Anderson, Masayuki Kikuchi, Takeshi Mikumo, Larry Ruff, and Dave Wald. This paper was built upon many hours of discussions during the Coffee Break of the Seismological Laboratory. I thank all the participants. This research was partially supported by the U.S. Geological Survey Grant 14-08-0001-G1774. This paper is Contribution No. 5214, Division of Geological and Planetary Sciences, California Institute of Technology.